Source : Webster's Revised Unabridged Dictionary (1913)

Buckle \Buc"kle\ (b[u^]k"k'l), v. i.
   1. To bend permanently; to become distorted; to bow; to curl;
      to kink.

            Buckled with the heat of the fire like parchment.
                                                  --Pepys.

   2. To bend out of a true vertical plane, as a wall.

   3. To yield; to give way; to cease opposing. [Obs.]

            The Dutch, as high as they seem, do begin to buckle.
                                                  --Pepys.

   4. To enter upon some labor or contest; to join in close
      fight; to struggle; to contend.

            The bishop was as able and ready to buckle with the
            Lord Protector as he was with him.    --Latimer.

            In single combat thou shalt buckle with me. --Shak.

   {To buckle to}, to bend to; to engage with zeal.

            To make our sturdy humor buckle thereto. --Barrow.

            Before buckling to my winter's work.  --J. D.
                                                  Forbes.